1. Field of the Invention
The present invention relates to a method of producing a maleimide-olefin copolymer having excellent transparency.
2. Description of the Related Art
A maleimide polymer, mainly phenyl maleimide, has been investigated as a polymer material having high heat resistance for many years. However, since the polymer product becomes remarkably brittle as the number of maleimide units is increased, the maleimide polymer is generally used only as a heat resistance modifier for ABS by adding in an amount of several tens %. In addition, the use of phenyl maleimide significantly causes coloring of the polymer product, thereby causing a critical problem according to the application.
A copolymer of an alkyl maleimide and a specified olefin has practical mechanical strength in spite of its high maleimide content, and is an interesting material having excellent heat resistance. However, since the copolymer product is easily colored by the effects of remaining monomers, impurities and the like, a copolymer having excellent optical properties cannot be easily industrially produced.
Generally known methods of radical polymerization of vinyl monomers include a suspension polymerization method, an emulsion polymerization method, a solution polymerization method and the like. The suspension polymerization method and emulsion polymerization method are frequently used in the industrial field.
However, when one of the above polymerization methods is employed for polymerizing a maleimide and an olefin, the problems below are produced. Since many maleimides are solid compounds at room temperature and are slightly soluble in olefins, it is difficult to employ a polymerization method such as the suspension polymerization method, the emulsion polymerization method or the like, which is generally applied to liquid monomers. In addition, since the above polymerization methods use a dispersion stabilizer or an emulsifying agent, the methods are undesirable from the viewpoint of deterioration in the optical properties of the produced polymers.
Further, in the case of a maleimide polymer, there is the problem that if maleimide or impurities remain in the produced polymer, as described above, the transparency and color tone of the polymer significantly deteriorate. From this point of view, the above polymerization methods are unsatisfactory production methods.
Although a homogeneous solution polymerization method is a production method suitable for polymerization of solid monomers, the polymerization method also has the following problems. For example, the produced polymer can be purified by precipitating from the polymer solution produced by reaction using a poor solvent to obtain a copolymer having a relatively good color tone. However, this method is impractical because of the problems that the produced polymer cannot be easily industrially purified by the above reprecipitation method, and that the production process becomes complicated. In addition, although a method of removing the solvent from the obtained polymer solution by distillation, flushing or the like is a relatively simple method, the method has the problem of significantly coloring the produced polymer.
As described above, it has been so far difficult to industrially produce a maleimide-olefin copolymer having excellent transparency and color tone.